In the magneto optical recording method, a substrate made of glass, plastic, ceramic or other material and coated with a vertically magnetized film composed of metal magnetic material serves as a recording media, and recording and reproducing operations on and from the recording medium are carried out in the following process.
In the recording operation, first the magnetization direction of the vertically magnetized film of the recording medium is arranged to a predetermined direction (upward direction or downward direction) by using a strong external magnetic field or the like (hereinafter this process is referred to as initialization). Then the temperature of a recording portion where the recording is to be carried out is raised to exceed the vicinity of the Curie point or is raised to exceed its magnetic compensation point by projecting a laser beam on the recording portion. As a result, the magnetic coercive force (Hc) on the recording portion becomes zero or substantially zero. With this state, the magnetization direction is reversed by applying an external magnetic field (bias magnetic field) whose magnetization direction is opposite to the initialized magnetization direction. When the projection of the laser beam is stopped, the recording portion of the recording medium returns to room temperature. Thus, since the reversed magnetization direction is fixed, information is recorded. The above-mentioned recording operation is also called thermomagnetic recording.
In the reproducing operation, a linearly polarized laser beam is projected onto the recording medium. The rotating direction of a polarization plane of reflected light or transmitted light from or through the recording medium varies according to the magnetization direction (upward or downward) of the recording medium (this is called magnetic Kerr effect or magnetic Faraday effect). Information is optically read out by the use of the magnetic effect.
Recording media used in the magneto-optical recording method have been noted as large capacity memory elements of a re-writable type. At present, there are two methods for re-writing over the information recorded on the recording medium, described in (i) and (ii) below.
(i) A method wherein the previously recorded information is deleted by initializing the recording medium again. PA1 (ii) A method wherein a recording medium or an external magnetic field generating device is improved so that overwriting is performed, i.e. the information is re-written directly without performing the deletion.
If method (i) is adopted, either an initialization device or two heads must be installed, thereby causing a rise in cost. Moreover, in case of deleting information with a single head, the same time taken for recording is required for deleting, resulting in the inefficient operation of re-writing information.
In the mean time, if method (ii), improving the recording medium, is adopted, it is difficult to control recording medium, composition, film thickness and so on. For the above reasons, the most effective means is improving the external magnetic field generating device of method (ii), i.e. switching a direction of the external magnetic field at high speed according to information signals while keeping the intensity of the laser beam constant.
In order to switch the direction of the external magnetic field at high speed, a magnetic head (a coil and a coil core) of the external magnetic field generating device must be miniaturized to a great degree. In this case, however, a generating area of the magnetic field becomes smaller. In order to counteract this, a magnetic head and a recording medium must be brought closer to each other. Thus, as shown in FIG. 10(a) and FIG. 10(b), generally a floating head 32 of a sliding type which can glide over a recording medium in the shape of a disc (not shown) is employed as the external magnetic field generating device. The floating head, 32 comprises a slider section 33 provided with a magnetic head section 34. The floating head 32 is pressed down toward the recording medium and supported by a suspension 31. According to the configuration, when the recording medium is rotated, the floating head 32 floats over the surface of the recording medium.
A constant floating height of the floating head 32 is maintained due to the fact that floating force balances with depressing force. The floating force is exerted upwards on the slider section 33 by the air flowing between the slider section 33 and the recording medium. The depressing force is exerted downwards on the slider section 33 by the suspension 31. The floating head of this type is also used for conventional hard disks of magnetic recording/reproducing devices. In the case of the hard disks, the floating height is of a submicron order. However, when the recording medium is a magneto-optical disk, a floating height of 5 .mu.m to 15 .mu.m is necessary, i.e. a larger floating height is needed for the magneto-optical disk than for the hard disk. The reasons for this are as follows. Since the magneto-optical disks are transportable, dust tends to stick more frequently on the disk. As a result, troubles such as a head crash, where the magneto-optical head 34 is damaged by dust as the floating height is too small, may occur.
A surface of the magneto-optical disk which faces the floating head 32, is textured with fine and physical protrusions and recessions, preventing the surface of the disk from sticking to the floating head. As shown in FIG. 11(a) and FIG. 11(b), when forming the disk surface with the texture, a texture tape 36 having fine protrusions and recessions on the surface thereof is used. More precisely, when forming the disk surface with the texture, the texture tape 36 is pressed onto a magneto-optical disk 35 by a tape pressure roller 37 and then the magneto-optical disk 35 is rotated while feeding the texture tape in the direction of arrow C. In this case, since the rotating direction of the magneto-optical disk 35 is in parallel with the feeding direction of the tape 36, the protrusions and recessions are evenly formed on the magneto-optical disk 35 as shown by the two-dot long and two short dashes line of FIG. 12, substantially forming concentric circles.
As aforesaid, in the case of the magneto-optical disk, the floating height of the floating head is 5 .mu.m to 15 .mu.m, greater than a floating height required when a hard disk is used. Therefore, variations in the floating height greatly depend on the relative velocity between the magneto-optical disk and the floating head. The relation is shown in Table 1 below. Referring to Table 1, if depressing force F of the suspension is constant and the relative velocity increases by two times, the floating height increases by a substantial one and half times (here, the dimensions of the slider section are 6 mm.times.4 mm). Thus, in case the magneto-optical disk is rotated based on the Constant Angular Velocity method (hereinafter referred to as CAV method), the relative velocity is higher in outer parts of the magneto-optical disk than in inner parts thereof, resulting in a higher floating height in the outer parts. Consequently, a magnetic field intensity applied to the magneto-optical disk by the floating head varies at each radial locations on the magneto-optical disk, and therefore a problem arises, i.e. the recording operation with a conventional magneto-optical disk cannot be carried out under constant conditions.
TABLE 1 ______________________________________ (the relation between the relative velocity and the floating height) V F 10 m/s 20 m/s ______________________________________ 5 gf 6.5 .mu.m 10 .mu.m 10 gf 4 .mu.m 6.5 .mu.m ______________________________________ Where, F: depressing force due to the suspension V: headmedium relative velocity